1. Field of the Invention
The present invention relates generally to electrochemical devices and, in particular, to an electrolytic tilt sensor and a method for making same.
2. Background of the Art
Electrolytic tilt sensors were originally conceived for weapons delivery and aircraft navigation and are now used in applications such as oil rig leveling, bore-hole angling, construction laser systems, marine stabilization, automotive wheel alignment, seismic and geophysical monitoring, virtual reality systems, and robotic manipulators, etc.
Electrolytic tilt sensors provide output voltage proportional to tilt angle and a phase indicative of tilt direction when configured in an appropriate electrical circuit. An electrolytic tilt sensor typically includes a glass envelope or non-conductive housing that is partially filled with an electrolytic solution, and a plurality of conductive electrodes, including one common electrode, which are at least partially immersed in the electrolytic solution. A portion of the cell which remains unfilled defines a gaseous bubble, which shifts as the cell is tilted, also causing the electrolyte to shift. Consequently, the electrodes become more or less immersed by the electrolyte as the bubble shifts. This shift provides a change in electrode coverage area which results in impedance change between any one electrode and the common electrode. When the tilt sensor electrodes are configured as part of an appropriate electrical circuit, the angle of tilt may be correlated to an output voltage of the circuit.
The geometric configuration of the enclosure and the arrangement and shape of the electrodes affect the quality and performance of each tilt sensing device, including linearity and sensitivity of the output signal. Various geometric configurations of enclosures of these devices have been disclosed in the prior art; for example, U.S. Pat. No. 2,713,726 discloses a rectangular enclosure; U.S. Pat. No. 3,487,303 discloses a spherical housing; U.S. Pat. No. 3,823,486 discloses a housing having a toroidal configuration; and U.S. Pat. No. 5,170,567 discloses a cylindrical housing.
A conventional electrolytic tilt sensor for use in a relatively narrow operating range (i.e., “narrow angle” tilt sensor) is illustrated in FIG. 1. The narrow angle tilt sensor 10 includes a tubular or “banana shaped” glass envelope housing 12, active electrodes 14a, 14b and common electrode 14c extending into the interior of the envelope 12, an electrolytic solution 16 surrounding at least a portion of the electrodes 14a, 14b and all of common electrode 14c, and a vapor bubble 20. There is typically a hermetic seal between the envelope and the electrodes, so that the electrolytic solution and the vapor bubble remain completely within the envelope.
The vessel configuration, electrolyte, electrode arrangement and number of electrodes may be varied to provide the desired operating characteristic. The electrodes may be comprised of platinum, and the electrolyte may be a solution of potassium iodide dissolved in ethanol. The curvature M of the housing may determine the operating and sensitivity range of the device. Alternating current (“AC”) is applied to electrolytic tilt-sensors. FIGS. 2 and 2A show two alternative typical electrical circuits to be used with his tilt sensor.
Known shortcomings associated with conventional envelope tilt sensors include difficulties in manufacturing the sensor. Moreover, tilt sensor manufacture requires a significant degree of skill, fixturing, and labor intensive handwork and art on the part of the operator to achieve the desired parameters. The tolerances of the glass housings during the process can vary greatly. This results in either a higher reject rate and/or a greater range of mechanical and electrical tolerance in the parts. In addition, the tilt sensor components are relatively fragile due to their glass construction and must be handled with caution. They are also costly to manufacture and generally use precious metal electrodes.
Electrolytic tilt sensors with conductive envelopes, such as metallic or partially metallic envelopes, have been disclosed in the prior art (see, for example U.S. Pat. No. 5,630,280 to Crossan, Jr. and U.S. Pat. No. 6,249,984 to Barsky, et al.). The Crossan, Jr. tilt sensor (U.S. Pat. No. 5,630,280) includes four arcuate sensing electrodes extending into a spherical chamber defined by the enclosure. The enclosure includes a metallic containment housing and a header made of a non-conductive material, such as glass. The metallic containment housing functions as the common electrode while the header secures the sensing electrodes and insulates them from the metallic containment vessel.
The Crossan, Jr. tilt sensor is for use in a wide operating ranges and provides certain advantages, such as enhanced linearity of output voltage resulted from arcuate electrodes in association with the spherical chamber.
The tilt sensor disclosed in Barsky, et al. (U.S. Pat. No. 6,249,984), the entire contents of which are herein incorporated by reference, includes a metallic envelope, a metal header welded to the envelope, and a plurality of electrodes located within the envelope. Each of the electrodes has a general straight configuration and is vertically mounted through the header while they are insulated by glass to metal seals. Similar to Crossan, Jr., this tilt sensor is also for use in a relative wide range of tilt angle.